The present invention relates generally to methods for manufacturing refractory blocks used in lining a furnace in which is conducted a process which produces slag, and more particularly to a method for treating a refractory block to improve its resistance to slag penetration.
In a typical manufacturing operation for forming a refractory block, a mixture of coarse and fine grains of uncured refractory material are initially formed into a block consisting essentially of relatively coarse grains in a matrix of relatively fine grains located in the interstices between the coarse grains. The fine grains are composed of a refractory material which is either the same as the refractory material of which the coarse grains are composed or of a refractory material which is compatible with the refractory material in the coarse grains, or both. A refractory material is compatible with another refractory material if it does not flux or reduce the melting point of the other refractory material during high temperature operating conditions.
The uncured block of refractory material has pores extending from the surface of the block inwardly along the matrix in the interstices between the coarse grains.
After formation, the uncured block of refractory material is subjected to a sintering operation to form a fully cured block of refractory material. The fully cured block is less porous than the unsintered block, but it still has some pores extending from the surface of the block inwardly along the matrix, and, during operating conditions in a furnace, slag can enter the pores in the fully cured sintered refractory block. These pores have cross-sectional spaces which are sufficiently large to permit penetration by the slag.
When a refractory block is penetrated by slag, the refractory block undergoes deterioration for a thickness substantially equal to the depth of slag penetration. More particularly, slag which has penetrated into the pores of the refractory block reacts with the refractory in the interior of the block, creating a new mineral different in composition from the rest of the refractory block. This new mineral expands and contracts at a rate different than the rest of the refractory block, creating a condition known as spalling in which layers of refractory material one to two inches thick flake off from the refractory block. Spalling reduces the effectiveness of the refractory block and reduces the life of the refractory lining composed of such refractory blocks, and this is undesirable.
Normally, refractory blocks are provided with a composition which has a minimal reaction with the slag, but such reactions can never be eliminated completely because, in order to do so, the refractory would have to be composed of material exactly the same as the slag, and this would cause the refractory block to become molten during operation of the furnace. Moreover, the slag from the furnace processing operation varies in composition during processing, so that a refractory composition which has a minimal reaction with a slag of one composition could have increased reaction when the slag undergoes a change in composition.
Among the prior art, Eusner, et al. U.S. Pat. No. 2,792,214 teaches that the rate of refractory deterioration can be reduced by reducing the porosity of the refractory block. Eusner discloses impregnating the refractory block with molten metal to fill the pores of the block and then oxidizing the metal to form a refractory metal oxide.
Church, et al. U.S. Pat. No. 4,077,808 teaches impregnating a ceramic block, composed of alumina, with phosphoric acid and then heating to react the phosphoric acid with the alumina to produce a complex aluminum phosphate which fills or partially fills the pores in the ceramic block, to harden and strengthen the ceramic block.
The ceramic block, thus hardened and strengthened, is used for mechanical or structural purposes. Church teaches that the ceramic block must be in an uncured or unsintered state in order to accomplish the aims of the Church procedure. If the ceramic block is fully cured before it is subjected to Church's impregnating step, the block is insufficiently porous to accomplish the aims of the Church procedure. Church also contemplates subjecting the ceramic block to an impregnating treatment with a solution of a metal salt which, upon heating, can be converted into a metal oxide, to fill or partially fill the pores of the uncured ceramic block, but this impregnating step must be performed before the step in which the ceramic block is impregnated with phosphoric acid or, according to Church, the treatment will not work. According to Church, a procedure in which impregnation with phosphoric acid precedes impregnation with a salt solution convertible to an oxide, has been found to produce a block retaining a high degree of porosity.
In describing the importance of assuring that the ceramic block is uncured when it undergoes the impregnation treatments described in the Church patent, Church states that the production of strongly bonded refractory materials has been found to require the presence of small pores, gaps, cracks or interstices and that, if these spaces are too small, then the chemical solution cannot penetrate properly.